This invention relates to an ink-jet printer that prints by discharging ink at the printing surface from a nozzle head with a large number of ink-jet nozzles arranged in the printing direction.
One known serial ink-jet printer is such that each color nozzle head with an ink cassette is moved back and forth over the full length in the direction of main scanning, characters and graphics are printed line by line (or 1/N line at a time) on a printing medium, such as ordinary paper or OHP paper, during the back-and-forth movement of the nozzle head, the printing medium is fed one line in the direction of feed after one line has been printed, and those processes are repeated until a specific number of lines have been printed. Furthermore, for example, Jpn. Pat. Appln. KOKAI Publication No. 10-138520, filed by the applicant of the present invention, has disclosed an ink-jet printer which not only prints much faster than the above serial ink-jet printer but also enables the continues printing of a large number of sheets of paper and which can be made much smaller than a laser printer of the electrophotographic type.
With the above printer, the printing medium transferred from the paper feeder section is conveyed by the convey mechanism to a rotating body with a specific timing, which winds the medium around its outer surface (printing surface). The rotating body is rotated at, for example, 120 rpm on its central axis in the direction of feed.
The printer is provided with a nozzle unit. On the nozzle unit, there are provided nozzle heads of cyan C, yellow Y, magenta M, and black B which extend in the direction of axis of the rotating body and are arranged in the direction of feed, or the direction of rotation of the rotating body. Ink is supplied from an ink supply mechanism including an ink tank and a supply pump. The nozzle heads shoot ink of the respective colors at the printing medium.
While a reciprocating mechanism is moving the nozzle unit back and forth a specific distance (for example, a distance equivalent to the pitch between ink-jet nozzles) in the direction of main scanning, line printing is done on the rotating body in operation and at the same time, column printing is done in the direction of feed, making use of the rotation of the rotating body. Namely, row printing and column printing can be done simultaneously making use of the high-speed rotation of the rotating body, which makes the printing much faster (for example, at 20 ppm). After one page of print has been finished, the nozzle unit is returned to the original position. Thereafter, it moves back and forth to print the next page. The printing medium on which printing has been done is peeled from the rotating body and let out by an eject mechanism.
In the printer, since the nozzle heads of the individual colors are provided away from the ink tanks of the respective colors, the nozzle head of each color can be made lighter, which enables the reciprocating speed in the direction of row to be increased remarkably. This makes it possible to increase not only the printing speed more but also the capacity of the ink tank of each color remarkably. As a result, for example, more than 500 sheets of paper can be printed continuously.
To print characters and graphics continuously onto a larger number of sheet of the printing medium as described above, it is desirable that the nozzle head of each color should be maintained, for example, to prevent the head from clogging up, at regular intervals of, for example, three hours. To make such maintenance possible, the nozzle unit is so provided that it can move in the horizontal direction between the printing position where the nozzle unit can print on the rotating body and the non-printing position where the nozzle unit is separated a specific distance away from the rotating body. After the nozzle heads have been moved to the non-printing position by the head moving mechanism including a motor and a rack, each nozzle head is maintained using a maintenance mechanism provided between the nozzle heads and the rotating body. After the maintenance, the head moving mechanism returns the nozzle unit to the printing position and the gap between the tip of each nozzle head and the printing surface is kept at, for example, about 1 mm.
It is difficult to produce the gap between the nozzle heads and the printing surface quickly and accurately at the printing position. Since the gap is directly related to the quality of print, the moving mechanism for the nozzle unit must be processed and assembled very accurately. As a result, the configuration of the printer as a whole is complex, which is liable to raise cost.
Even when the nozzle unit has been provided in the printing position very accurately, or the head moving mechanism has been locked after the positioning, if the head moving mechanism transmits the vibration of the motor to the nozzle unit side or if there is looseness in the component parts of the head moving mechanism including a motor, it is difficult to keep the gap accurately and stably without any fluctuation.